Hey Chewy!
I just got an Eagle 320c.
RAM makes a tubing mount for it, but no one locally had it. Plus ram mounts are pricey. SO.....
one old plastic cutting board
2 u-bolts
saw
drill
----
I cut the plastic into a circle on a band saw. Then I used a milling machine to cut 2 curved slots, to fit the underside of the Eagle 320 screw holes. This lets the base of the Eagle swivel if needed.
I clamped the U bolts onto the pontoon frame, just under the left side oarlock, and drilled thru the plastic for them.
Presto.....that's it.
I'd snap a pic for ya, but the dang boat is under over a foot of fresh snow right now!'

For the transducer.....I know I'll forget it was there and scrape the thing to pieces if I strapped it under a pontoon. So I took a piece of 1-1/2" PVC pipe with and end cap glued on, hole drilled in the end cap. Screwed the transducer tilt-out mount (pops back if you hit something) to the end cap. The pipe mounts to the aluminum back deck of my pontoon boat. (it's a Scadden Madison River). I can reach back and pop the whole pipe up when coming into shore. The wires run thru the hole an up the pipe.

Hope that helps.....DANBOB